1. Field of the Invention
This invention relates in general to measuring and testing and more particularly relates to an apparatus for measuring torque. Most particularly, the invention relates to a torque sensor for magnetically detecting torque transmitted to a rotatable shaft without directly contacting the shaft.
2. Description of the Prior Art
Torque sensors which magnetically detect torque through magnetoelastic and/or magnetostrictive phenomena are well known. There are typically two types of torque sensors. One type of torque sensor includes a rotatable shaft made of a magnetic alloy. Another type of torque sensor includes a rotatable shaft carrying a magnetic alloy layer on its outer peripheral surface. Either type of torque sensor further includes magnetic field sensors that are disposed adjacent the magnetic alloy. The magnetic field sensors are provided for detecting a magnetic field emanating from the magnetic alloy. The magnetic field sensors are typically, but not necessarily, configured and arranged to exclude influence of an external magnetic field, such as the magnetic field of the earth. For example, when a torque is transmitted to the rotatable shaft, the magnetic alloy is mechanically stressed or otherwise deformed. This causes a magnetic field to be emanating from the magnetic alloy. The magnetic field is sensed by the magnetic field sensors. The magnetic field sensed by the magnetic field sensors is measured by a detector, which produces an output signal that correlates to a direction and magnitude of the torque transmitted to the shaft.
When measuring torque in a rotatable shaft, ideally the emanating magnetic fields would be axisymmetrical. However, anomalies in the magnetic alloy and/or in the magnetic preconditioning imparted on the magnetic alloy can produce non-axisymmetrical inhomogeneities in the magnetic field emanating from the shaft. As the shaft rotates, the anomalies in the magnetic alloy move with the shaft. Consequently, the inhomogeneities in the magnetic field emanating from the shaft move as the shaft rotates. When the inhomogeneities in the magnetic field are sensed by the magnetic field sensors configured in an circumferentially limited angular expanse about the shaft, the detector produces a deviation in the output signal that is erroneously interpreted as a change in direction and/or magnitude of the torque transmitted to the shaft. The desired response of the sensor should be to applied torque only. By contrast, the anomalous output signal is associated with an undesirable response to the rotation angle of the shaft.
What is needed is a torque sensor that is unaffected by anomalies in magnetic alloy and thus provides an accurate and dependable detection of torque transmitted to the rotatable shaft.
Generally speaking, the present invention is directed towards a torque sensor that meets the foregoing needs. The torque sensor magnetically detects torque transmitted to a rotating rotatable shaft without directly contacting the shaft. The torque sensor comprises a magnetic alloy and one or more sensing elements about the magnetic alloy to measure a magnetic field emanating from the magnetic alloy without being affected by anomalies in magnetic alloy.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.